1. Field of the Invention
This invention relates generally to networks of data processing systems and, more particularly, to the coupling of local area network (LAN) systems. In the present invention, the coupling of the two LAN systems includes a wireless link.
2. Description of the Related Art
When a local area network system is envisioned, one of the first problems that must be addressed is the avoidance of data integrity compromise resulting from simultaneous or overlapping application of signal groups to the system communication channel by a plurality of the system components. Several techniques have been developed to address the problem of overlapping signal groups in the communication channel. According to a first technique, a system component is selected to apply signals to the communication channel while the other components assume the role of signal group receivers, Techniques such a "token rings" provide a flexible procedure for designating a (signal group) transmitting component and thereby avoiding the problem of overlapping signal groups in the communication channel (i.e., applied to the system bus). Other strict protocol techniques, such as "round-robin" protocol can designate the current system component permitted to apply signal groups to the system bus. Another technique for avoiding overlapping signal groups on a local area network system bus is to multiplex the system bus in the time domain and assign a time "slot" to each system component. Because the communications emanating from each system component can only occur at predetermined times, overlapping signal groups are avoided. A third technique permits the system components to apply signal groups to the communication channel in a generally random fashion subject to the restriction that the signal groups are not to be applied to the communication channel when the communication channel is busy. As will be clear, in this technique overlapping signal groups can be applied to the (apparently available) communication channel at substantially the same time, resulting in overlapping signal groups (i.e., a collision) and, consequently, in a compromise of signal group integrity. In this technique, the presence of overlapping signal groups, generally referred to as signal group `collisions`, is detected and the signal groups are reapplied to the communication channel until the transfer is successful (i.e., the signal transfer integrity has not been compromised by overlapping signal groups). This technique of LAN system implementation is generally referred to as a CSMA/CD (Carrier Sense Multiple Access/Collision Detect) system. The ETHERNET system of Digital Equipment Corporation is implemented in this fashion.
In order to increase the usefulness of LAN systems, techniques have been developed to couple two LAN systems together. Referring to FIG. 1, the general configuration of such a coupling is illustrated. A first local area network 10 includes a LAN system bus 11 over which information is transferred by data processing or communication systems coupled to the system bus 11. Similarly, local area network 10' includes a system bus 11' over which information is transferred by data processing system or communication systems coupled to the system bus 11'. A bridge device 15 is coupled to system bus 11 and to system bus 11'. The function of the bridge device 15 is to identify those signal groups that on the system bus of a first LAN system that are intended for distribution on the system bus of the second LAN system and to transfer those signal groups to the second LAN system bus. For the selected transmitting component technique or the time domain multiplexing technique, the bridge device 15 can be implemented as another system component, a system component synchronized with the two coupled LAN systems. The bridge 15 device for CSMA/CD type LAN system is somewhat more complicated because, for example, a signal group thought to be successfully transmitted by a component of the first LAN system can encounter a collision in the second LAN system. Therefore, in the bridge devices provided for CSMA/CD LAN networks, separate transmission apparatus is provided for transmitting signal groups in either direction. In addition, temporary signal storage apparatus is provided for both sets of transmission apparatus to store signal groups successfully transmitted from a first LAN system, but encountering a collision in the second LAN system. The storage apparatus permits the signal group, compromised by a collision in the second LAN system, to be reapplied to the second LAN system from the bridge device 15 rather than from the originating component in the first LAN system.
A pair of LAN systems can be so positioned that it is impractical to provide a physical communication channel, such as a bus, to couple the systems. If a bridge network that included a wireless link were available, then the communication between the LAN systems can be implemented. For bridge networks that couple non-CSMA/CD LAN systems, the presence of the wireless link can be accommodated by extension of the collision avoidance techniques used by the LAN systems. However, in coupling the CSMA/CD LAN systems the problem of the overlapping signal groups is more severe because of the lack of a physically available channel of communication, especially for the detection of a collison between signal groups.
A need has therefore been felt for a technique of coupling two CSMA/CD LAN systems by a bridge network that includes a wireless link, the bridge network being capable of identifying and responding to simultaneous exchange of information signals over the wireless link.